In an active matrix display device such as a liquid crystal display device or an organic electroluminescence (EL) display device, a thin-film transistor (TFT) is widely used as a switching element or a driving element.
Recent years have seen active research and development of TFTs using an oxide semiconductor, such as zinc oxide (ZnO), indium gallium oxide (InGaO), indium gallium zinc oxide (InGaZnO), or the like for the channel layer. For example, Patent Literature (PTL) 1 discloses a bottom-gate thin-film transistor using an oxide semiconductor. A TFT using an oxide semiconductor for the channel layer has characteristics of a small OFF current, high carrier mobility even in an amorphous state, and being able to be manufactured in a low-temperature process.